Illumination unit for the generation of optical sectional images in transparent media, in particular in the eye

ABSTRACT

Illumination unit for the generation of optical sectional images in transparent media, particularly in the eye.  
     In the arrangement according to the invention, the low-divergence beams emitted by a laser serving as illumination source are imaged on or in the eye under examination by a reflection element which is controllable in a defined manner and beam deflection elements present in the beam path. The optical sectional images resulting in and on the eye can be observed and/or recorded, further processed and evaluated with an image processing unit in a known manner.  
     In the solution according to the invention, a sectional image is generated by the deliberate periodic beam deflection of a particularly fine laser beam with high depth of focus, which sectional image remains sharp through the entire dimension of the object to be examined and makes possible an improved evaluation. The intensity of the laser beam bundle can be varied in such a way that it is sufficient for observation and documentation, but so that the diameter of the beam bundle is fine enough for a high detail resolution.

[0001] The present invention is directed to a special illumination unit for generating a variable slit image such as is used in ophthalmological examination instruments, including slit lamps. A light section is generated by slit image projection in the more or less transparent object under examination, e.g., in the interior of the eye. The parameters of the section bundle are variable, particularly the angle of incidence, the dimensions of the slit image, its intensity and the spectral composition. Information about the object under examination can be acquired from the shape, position and intensity of the scattered light of the sectional image generated in this way.

[0002] In slit lamps such as those described in Rassow, B. et al., “Ophthalmologic-optical instruments”, 1987, Ferdinand Enke Verlag Stuttgart, pages 99ff and 137ff, mechanical/optical elements such as slit diaphragms have been used up until now to generate slit imaging. The variable, smallest possible slit widths which are required for high optical detail resolution within the optical section are very difficult to achieve. Further, the mechanical component groups are very cumbersome to adjust, and this is made even more difficult by the thermal expansion of the component groups, among other things. Reproducibility of adjustments for measurement purposes is possible only to a limited extent. The multiplicity of possible illuminated field geometries is extremely limited by the fixed slit diaphragms and pinhole diaphragms and the space required by the latter.

[0003] Since slit image projection involves optical imaging with a physically defined depth of focus, the imaging must always be focused strictly on the point of examination. A section bundle which is focused over the entire extension of the human eye can not be achieved with the solutions mentioned above. While conditions can be improved with an arrangement based on the Scheimpflug principle, the technical expenditure is correspondingly greater.

[0004] DE 198 12 050 A1 describes a method and an arrangement for illumination in an ocular microscope. A wide variety of illuminating mark geometries is generated by means of opto-electronic components. The illuminated field geometries are projected on the anterior portion and fundus of the eye and are used for general examination of the eye.

[0005] Methods and arrangements for illuminating the anterior segments of the eye in which a planar configured laser is used as light source are described in U.S. Pat. Nos. 5,404,884, 5,139,022 and 6,275,718. However, in these solutions the limited variability of the slit dimensions, the utilized wavelength of the laser sources, and the lack of a possibility for generating multiple-slit projections are disadvantageous under certain circumstances. The described arrangements are not slit lamp devices used in normal diagnostic operation. Further, the system for receiving the scattered light from the eye has a physically limited depth of focus which can not completely acquire the expansion area of the sharp sectional laser image.

[0006] It is the object of the present invention to further develop the known solutions in such a way that a sufficient light intensity for observation and documentation, particularly with analog or digital image acquisition technology, can be ensured in spite of the smallest possible slit width and greatest possible depth of focus which are required for high optical detail resolution within the optical section.

[0007] According to the invention, this object is met through the characteristic features of the independent claims. Preferred further developments and embodiments are indicated in the dependent claims.

[0008] With the proposed solution for an illumination unit for generating optical sectional images in the eye, a sectional image is generated by the deliberate periodic beam deflection of a particularly fine laser beam with high depth of focus, which sectional image remains sharp through the entire dimension of the object to be examined and makes possible an improved evaluation. The intensity of the laser beam bundle can be varied in such a way that it is sufficient for observation and documentation, but so that the diameter of the beam bundle is fine enough for a high detail resolution. The proposed illumination unit could be constructed in such a way that it is used in addition to an existing illumination unit of an ophthalmological instrument such as a slit lamp. As an add-on module for existing ophthalmological instruments, it could substantially simplify examinations of the human eye through broad application and improve the accuracy of the examination results.

[0009] The invention will be described in more detail in the following with reference to an embodiment example.

[0010]FIG. 1 is a schematic view of a slit lamp with the illumination unit according to the invention; and

[0011]FIG. 2 is a schematic view of a microscanner mirror.

[0012]FIG. 1 shows the construction of a slit lamp with the illumination unit, according to the invention, for generating optical sectional images. The low-divergence beams proceeding from the laser serving as illumination source 1 are imaged on or in the eye 6 under examination by a reflection element 4 which is controllable in a defined manner and beam-shaping and beam-deflection elements (shown schematically) in the beam path. The angle and the direction of the laser beam 3 directed onto the eye under examination are freely selectable.

[0013] A laser diode module, for example, is used as illumination source, preferably in the blue or green spectral range. Illumination with light in the green or blue spectral range results in higher scattering in the transparent media of the eye, which leads to clearer sectional images for optical diagnosis. A very fine, low-divergence laser beam 3 is generated by means of focusing optics 2. This laser beam 3 impinges at a determined angle and distance on the reflection element 4 whose frequency and amplitude can be controlled by the control unit 7. The reflection element 4 is a mirror which has small dimensions and which is tiltable in two or more directions independent from one another. This so-called microscanner mirror chip preferably belongs to the group of MEMS (microelectro-mechanical systems) and can be constructed in any manner desired. A chip of this type is available, for example, from Fraunhofer-Institut fur Mikroelektronische Schaltungen und Systeme Dresden under the trade name “Resonanter 1D- und 2D-Mikroscannerspiegel”.

[0014] Also, a DMD (digital mirror device) type microscanner mirror chip such as that available from Texas Instruments, for example, can be used as a reflection element.

[0015]FIG. 2 shows a schematic view of a MEMS type microscanner mirror chip. The actual mirror element 9 is tiltable along axes 11 and 12 by means of the holder frame 10. Tilting around the respective axes 11 and 12 can be carried out, for example, by generating harmonic mechanical oscillations of suitable frequency and amplitude by means of electrostatic control. The microscanner mirror, which is fastened by torsion springs or by a universal joint suspension, is accordingly set in harmonic mechanical oscillations. In this way, many different shapes can be imaged on or in the eye under examination from the punctiform low-divergence laser beam 3. When the microscanner mirror oscillates in only one direction, for example, the image of a slit is generated.

[0016] In the microscanner mirror chip produced by Fraunhofer-Institut, the adjustable oscillation frequencies are between 150Hz and 20kHz depending on the image to be produced. The amplitude of the oscillation is directly proportional to the drive voltage in this microscanner mirror chip and can be regulated by means of this drive voltage. Deflecting angles of up to 60° can be achieved by the electrostatic drive principle.

[0017] In principle, electrostatic, thermomechanical, piezoelectric and magnetic excitation forms are possible for the control of the microscanner mirrors. A large number of displayable images, e.g., single slits and multiple slits, grids and rasters, result from the many possibilities of modulation with respect to frequency, amplitude and intensity.

[0018] A particular advantage of the illumination unit according to the invention consists in the possibility of a quasi-simultaneous imaging of a plurality of slit images on or in the eye under examination. In this way, the examination period can be shortened and the physical stress on the patient can be substantially reduced.

[0019] The laser beam 3 which can be changed in this way with respect to its propagation direction and also with respect to its intensity by additional modulation is deflected on or in the eye 6 under examination by the beam-shaping and beam-deflecting element 5.

[0020] The optical sectional images resulting in the eye 6 can be observed in the usual manner, e.g., with a variable-magnification stereo microscope or similar arrangement. Current ophthalmological instruments generally have, in addition to an observation system, an image processing unit by means of which the sectional images can be recorded, further processed and evaluated.

[0021] Because of its small dimensions, the illumination system according to the invention can be integrated in many different ophthalmological examination instruments, especially slit lamps. The illumination unit can be combined with the conventional, existing illumination arrangement or can be used separately. It is also possible to use the illumination unit as an add-on module for expanding existing ophthalmological instruments in order to substantially simplify examinations of the human eye and to improve the accuracy of the examination results. Modern ophthalmological examination instruments have additional documentation devices such as photo/video components for analog and digital image recording and image processing and an automatic image evaluation for obtaining measurement values of the object under examination. In order to increase the depth of focus during documentation, the recording unit with its receiver (e.g., CCD chip) can be inclined relative to the optical axis so as to be adapted to the angle of the slit input radiation direction corresponding to the Scheimpflug condition. The effect can be monitored with a monitor image in an advantageous manner.

[0022] In the illumination unit according to the invention, the intensity of the laser source can advantageously be selected in such a way that it is sufficient for observation and documentation, but such that the diameter of the beam bundle is fine enough for a high detail resolution and limiting values for the spectral irradiation strength on the fundus of the eye are not exceeded. Further advantageous effects, e.g., spatial separation of projection structures, can be achieved by means of a suitable modulation of the intensity of the light source. For this purpose, current laser diode modules generally have an additional connection for applying a modulation signal. Different illumination structures such as point grids and line grids or multiple-slit images can be realized in this way. These illumination structures can likewise be generated as dynamic processes in order to automate certain sequences or processes.

[0023] The described illumination unit serves to generate optical sectional images not only in the eye but also in different transparent media. The illumination unit can also be used, for example, for examination of liquid layers and/or for testing optical components such as lenses, prisms, and so on. It can be advantageous that the microscanner mirror is not set in oscillations but rather executes a slow scanning movement over the object to be tested. Since not all microscanner mirrors offer the possibility for a control of this kind, an appropriate model must be selected. 

1. Illumination unit for generating optical sectional images in transparent media, particularly in the eye, characterized in that a laser is provided as illumination source (1), and in that there is a reflection element (4) which is controllable in a defined manner for the deliberate deflection of the laser beams.
 2. Illumination unit according to claim 1, characterized in that the beam emitted by the laser serving as illumination source (1) has a low divergence, and/or in that the beam emitted by the laser is preferably in the blue or green spectral region, and/or in that the intensity of the illumination unit (1) is controllable, and/or in that the illumination unit (1) has beam-shaping elements (2).
 3. Illumination unit according to claims 1 and 2, characterized in that the illumination source (1) is a laser diode module.
 4. Illumination unit according to claims 1 to 3, characterized in that the reflection element (4) is controllable with respect to frequency and amplitude, in that the reflection element (4) is a mirror of small dimensions, in that the reflection element (4) is tiltable in at least one direction, preferably in two or more directions independent from one another, and in that the reflection element (4) is a microscanner mirror chip of any type of construction.
 5. Ophthalmological examination instrument, particularly slit lamp, characterized in that it contains an illumination arrangement according to claims 1 to 4, in that the illumination unit (1) is used in combination with the conventional illumination device and can be switched on when needed, or in that the illumination unit is used separately from the conventional illumination device and can be added optionally, in that additional documentation devices such as photo/video components for analog and digital image recording and image processing are provided, in that there is an additional automatic image evaluation for obtaining measurement values of the object under examination, and in that the documentation device can be tilted at an angle for increasing the depth of focus according to the Scheimpflug condition so as to improve acquisition of the spatial extent of the sectional image through its entire dimension. 